Tight Space Calls for Creative Pipe Design at UV Disinfection Site

When the City of Cincinnati took the voluntary step of adding Ultraviolet (UV) Disinfection to its Richard Miller Treatment Plant, a tight space necessitated careful planning of pipelines and the flexibility to make ongoing adjustments in the field.

Greater Cincinnati Water Works is currently the largest water utility in North America to add UV disinfection and the only municipality with three-stage treatment. The $30 million, 19,600-square-foot UV disinfection facility, set for completion next year, will provide an additional barrier against cryptosporidium and giardia, should that be necessary in the future, and also ensures compliance with LT2ESWTR.

To accommodate the property's hillside and alluvial flood plain soil, engineers installed the UV facility on 151 auger-cast friction-based piles; the original plan was shifted 5 feet to provide additional room for the pipelines.

Accommodating the new UV facility alongside the existing sand filtration and granulated activated carbon (GAC) adsorption contactors left just ¼ acre under an existing parking lot in which to lay 600 feet of new 78-inch-diameter pipes.

Prestressed concrete cylinder pipe (PCCP) from Hanson Pressure Pipe was employed to re-route twin lines from the GAC treatment building to the new UV facility and then to existing clear well storage. The E-301 (AWWA C301) prestressed concrete embedded cylinder pipe is made by helically wrapping, under measured tension and at uniform spacing, a high tensile strength wire around the concrete core. The wire wrap places the concrete structural core and the embedded cylinder under compression, developing the pipe's ability to withstand specified hydrostatic pressures and external loads.

600 feet of 78-inch E-301 prestressed concrete cylinder pipe from Hanson Pressure Pipe was used to re-route twin lines from the existing GAC treatment building to the new UV facility and then to existing clear well storage.

PCCP is custom made for each application and is the only pipe material designed using the simultaneous application of internal and external pressure loads. Encasing the steel cylinder in highly alkaline Portland cement concrete electrochemically protects it from corrosion, and the concrete continues to gain compressive strength over time.

Hanson's custom approach to fabrication and hands-on engineering support were critical to solving the project's logistical challenges. Consulting engineers CDM Smith, consulting site engineers RA Consultants, and Hanson engineers worked together to determine the best layout for pipe lengths, joints, fittings, and valves to not only fit within the ¼-acre space but also to enhance field constructability. A significant number of custom fittings, including compound and 90-degree bends, were used to negotiate the final layout.

Because the new pipes won't be brought online until well after installation and because the pipe segments are not independent of the existing lines, the engineering team had to help determine how to hydrostatically test the new pipes. Temporary internal dished bulkheads, installed within the UV disinfection building's foundation wall, allowed the contractor to isolate the new pipe segments for pressure testing. Once testing was completed, all but two of the bulkheads were removed.

The final two bulkheads will be removed when the General Contractor, Adams Robinson Enterprises, is ready to divert water into the new facility in Spring 2013. Removal of the bulkheads will again require coordinated sequencing to ensure continual operation of at least one existing 78-inch supply line at all times.